An archive file is any file with the archive attribute
turned on. Many files that you'll encounter in normal computer use will likely have the archive attribute turned on.
The archive attribute is usually turned on when a file is created or modified. Once the file is backed up by a backup software program, the archive attribute is turned off. This way, the backup program can use the archive attribute to determine which files have changed and need to be backed up and which have not changed and do not need backed up.
Examples: "The old backup program I used to have checked only for the archive attribute when determining what to backup. I found out later that some special files do not automatically add the archive attribute when changed so I upgraded to a program that also compared file dates when backing up."